Electrochemical cathode and materials therefor

ABSTRACT

There is disclosed a process for producing an electrochemical cathode for an electrochemical cell comprised of a current collecting layer or substrate laminated between layers of a nonwoven conductive fibrous web, preferably of conductive carbon fibers, impregnated with a mixture of carbon particles and a nonfibrous polymeric substance, and optionally with a hydrophobic microporous film or layer disposed on one of the layers of the nonwoven conductive fibrous web as well as the product produced thereby.

BACKGROUND OF THE INVENTION Related Applications

This is a continuation-in-part of application Ser. No. 07/070,183, filedJuly 6, 1987, U.S. Pat. No. 4,885,217.

Field of the Invention

This invention relates to a process for manufacturing electrodes andelectrodes produced thereby, and more particularly to a process formanufacturing electrochemical cathodes for use in electrochemical cellsand electrochemical cathodes produced thereby.

Description of the Prior Art

Metal/air batteries produce electricity by electrochemically coupling ina cell a reactive metallic anode to an air cathode through a suitableelectrolyte. As is well known in the art, an air cathode is a typicallysheetlike member having opposite surfaces respectively exposed to theatmosphere and to an aqueous electrolyte of the cell, in which (duringcell operation) atmospheric oxygen dissociates while metal of the anodeoxidizes providing a usable electric current flow through externalcircuitry connected between the anode and cathode. The air cathode mustbe permeable to air but substantially hydrophobic (so that aqueouselectrolyte will not seep or leak through it), and must incorporate anelectrically conductive element for external circuitry.

In present-day commercial practice, the air cathode is commonlyconstituted of active carbon (with or without an addeddissociation-promoting catalyst) containing a finely divided hydrophobicpolymeric material and incorporating a metal screen as the conductiveelement. A variety of anode metals are used including iron, zinc,magnesium, aluminum, alloys of aluminum, etc. Alloys of aluminum andmagnesium are considered especially advantageous for particularapplications owing to low cost, light weight and ability to function asanodes in metal/air batteries using neutral electrolytes, such as seawater or other aqueous saline solutions. Metal/air batteries have anessentially infinite shelf-storage life rendering them very suitable forstandby or emergency uses in that the metal-air battery may be activatedby immersing the electrode in an electrolyte.

In the aforementioned copending application Ser. No. 07/070,183, thereis disclosed an air cathode comprised of a sheetlike laminate includingfirst and second layers having opposed major surfaces, respectively,exposed for contact with a liquid electrolyte and with air and facingmajor surfaces. The second layer is permeable to air but not to a liquidelectrolyte. A current-collecting layer is in contact with the firstlayer and is connected to external electrical circuitry. The first layerof the cathode is comprised of a nonwoven fibrous web, preferably ofconductive carbon fibers, impregnated with a mixture of carbon particlesand a nonfibrous polymeric substance for holding the carbon particles inthe web.

The facing major surfaces of the first and second layers are bondedtogether by heat seal coating material distributed on facing majorsurfaces in manner to provide an array or network of areas free ofcoating material extending substantially uniformly thereover with thecoating material being distributed as a multiplicity of spaced-apartdots, or as a mesh having coating-material-free interstices.Coating-free spaces maintains sufficient unclogged pores in the secondlayer to enable the air cathode to function as intended, yet witheffective lamination of the layers to each other and/or to thecurrent-collecting means, such as a layer of metal mesh interposedbetween and coextensive with the first and second layer.

While such an air cathode have been effective there is a tendency todelaminate, and thus it is desirable to form air cathodes using readilyavailable raw materials, under facile processing condition where the aircathodes are more reliable for intended use with extended shelf life.

OBJECTS OF THE INVENTION

An object of the present invention is to provide an improved cathode forelectrocemical cells.

Another object of the present invention is to provide an improvedelectrochemical cathode for electrochemical cells providing improvedelectrochemical characteristics.

A further object of the present invention is to provide an improvedelectrochemical cathode for electrochemical cells exhibiting improvedresistance to delamination.

Still another object of the present invention is to provide a novelprocess for producing an improved cathode for electrochemical cells.

Yet another object of the present invention is to provide a novelprocess for producing an improved cathode for electrochemical cellsexhibiting improved electrochemical characteristics, such as improvedcorrosion resistance.

A still further object of the present invention is to provide a novelprocess for producing an improved cathode for electrochemical cells atnominal temperature ranges.

SUMMARY OF THE INVENTION

These and other objects of the present invention are achieved by aprocess for producing an electrochemical cathode for an electrochemicalcell comprised of a current collecting layer or substrate laminatedbetween layers of a nonwoven impregnated with a mixture of carbonparticles and a nonfibrous polymeric substance, and optionally with ahydrophobic microporous film or layer disposed on one of the layers ofthe nonwoven conductive fibrous web as well as the product producedthereby.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become morereadily apparent from the following detailed description when taken withthe accompanying drawings wherein;

FIG. 1 is a schematic elevational sectional view of an electrochemicalmetal/air battery incorporating an electrochemical cathode of oneembodiment of the present invention;

FIG. 2 is a schematic exploded view of the embodiment of anelectrochemical cathode of FIG. 1; and

FIG. 3 is a schematic exploded view of another embodiment of anelectrochemical cathode, such as for a lithium battery, of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a metal/air battery,generally indicated as 10 comprised of a housing 12 defining a chamber14 for receiving a liquid electrolyte 16, such as an aqueous solution ofsodium chloride; an air cathode, generally indicated as 18, and a metalanode 20. The anode 20 is connected by line 22 to a light bulb 24connected by line 26 including a switch 28 to the air cathode 18. Thehousing 12 defines a large vertical aperture 30 for receiving the aircathode 18 in liquid-tight fashion with the periphery of the air cathode18 sealed to the periphery of the aperture 30 of the housing 12. Closingof the switch 28 closes an electrical circuit formed between the aircathode 18 and the anode 20 via the electrolyte 16 thereby illuminatingthe light bulb 24.

The air cathode 18 is a laminate structure comprised of a metal meshsubstrate 32, reactive layers 34 laminated to the mesh substrate 32 anda hydrophobic microporous layer 36 mounted to the outer reactive layer34. The metal mesh 32, is the current collector of the metal/air battery10, and is formed of a suitable metallic material, such as nickel,stainless steel and the like, formed for example by expanded metaltechniques and includes an upwardly extending tab portion 38 forconnection to line 26.

The anode 20, may be formed of a suitable anodic material, such asaluminum, magnesium or the like, and is illustrated as being disposed ina top 40 of the housing 12 in the form of a plate member extendingvertically into the chamber 14 including the electrolyte 16. The anode20 is longitudinally disposed with reference to the housing 12 and isspaced apart in parallelled relationship to the air cathode 18 adistance sufficient to form a gap therebetween for the electrolyte 16.The general arrangement of the metal/air battery 10 may be substantiallythe same as that of one of the cells of the plural-cell battery, such asdescribed in U.S. Pat. No. 4,626,482 hereby incorporated by reference.

In FIG. 3, there is illustrated an electrochemical cathode, generallyindicated as 42, constituting another embodiment of the presentinvention and a laminate structure comprised of the metal mesh substrate32 laminated between an inner reactive layer 34 and an outer reactivelayer 34 without any hydrophobic microporous layer mounted to an outerreactive layer 34. Such embodiment of the present invention findsapplication in the lithium battery art.

The reactive layer 34 is comprised of a conductive nonwoven fibrous webimpregnated with a mixture of carbon particles and a nonfibrouspolymeric substance for holding the carbon particles in the web. Othersubstances e.g. catalyst, may also be included in the impregnatingmixture. The nonwoven fibrous web of the present invention is preferablyformed of electrically conductive carbon fibers having a length of fromabout 0.5 to 1.25 inches and a diameter of from about 5.0 to 15.0μ. Thecarbon content of such carbon fibers is preferably at least about 90weight percent exhibiting a resistance of less than about 20 ohms/sq.

The conductive nonwoven fibrous web is formed into a thickness of from 5to 20 mils. exhibiting a pore volume of at least about 90 percent, toprovide a high pick-up of the impregnation mixture whether in suspensionor in a coating format. The impregnation mixture should increase thebasic weight of the conductive nonwoven fibrous web by about 20 to 120g./m.². as a function of solids content of the impregnation suspensionor coating and pore volume of the conductive nonwoven fibrous web, andprovide a front-to-back (F/B) electrical conductivity of less than about1 ohm.

The impregnation mixture, in the form of a suspension or coating, iscomprised of carbon particles, generally in the form of carbon black, anonfibrous adhesive polymeric material to bind the carbon particles tothe web and a dispersion or mixing agents. The carbon particles arepreferably of small size, expressed as surface area (m.² /g.), generallyin excess of 1000, and preferably greater than about 1250, such as BlackPearls 2000, available from Cabot Corporation having a mean particlediameter of 0.015μ.

The nonfibrous adhesive polymeric materials include thepolytetrafluoroethylenes, such as Teflon® T-30, a registered trademarkof duPont. The dispersion or mixing agents include the sodium salt ofpolymeric naphthalene sulfonic acid and those included with thepolytetrafluoroethylene (PTFE).

The conductive nonwoven web of carbon fibers is impregnated byimmersion, coating extrusion or the like, with the aqueous suspension ofthe impregnation mixture of the carbon particles, nonfibrous polymericsubstance and other desired nonfibrous ingredients, e.g. a catalyst. Acatalyst, such as potassium permanganate, may be added to theimpregnation mixture to improve (or reduce) polarization voltage of theresulting cathode. Generally, potassium permanganate is added in anamount of from 8 to 20%, preferably above 12% based upon dried weight ofcarbon dispersion solids and dispersion agent. Silver oxide has alsobeen found to be an effective catalyst in the system of the presentinvention.

The impregnated conductive nonwoven fibrous web is dried at atemperature of from about 150° F. to 400° F., a temperature below thesintering temperature of the binder material, i.e. the PTFE, and atemperature high enough to ensure substantially complete moistureremoval.

The microporous or hydrophobic layer 36 is a film or web permeable toair but impermeable to the solvent of the electrolyte system to preventor minimize penetration of the electrolyte through the air cathode tothe exterior of the metal/air battery 10 with concomitant disappearanceof active interface sites. The hydrophobic layer 36 is laminated on thereactive layer 34 for intended use particularly in conjunction withliquid electrolytes. It has been found that a film of polymerichydrophobic material, such as films of polytetrafluoroethylene of athickness of from 2 to 10 mils embossed with mesh of a nettedconfiguration of 1/30th" to 1/8" diameter provides particularlydesirable hydrophobic properties while concomitantly providing foreffective diffusion of a gaseous fuel. Embossing of the polymerichydrophobic layer may be readily effected by passage through anembossing roller assembly maintained at a temperature of from 150° F. to300° F., generally at a temperature to compact the film at the contactpoints or areas of the embossing roll. Wire mesh used to form thecurrent collector layer has been found effective in embossing thepolymeric hydrophobic layer.

As disclosed in the aforementioned copending application, the layers ofthe air cathode laminate 18 are bonded by a discontinuous heat sealcoating applied to the reactive layers as a discontinuous dot matrixsystem or an interposed layer of meshed coating material to besubsequently heated and subjected to a pressure step during passage ofthe air cathode laminate through a pressure roller system (not shown).Bonding of a reactive layer to the metal mesh substrate is particularlyeffectively performed by passing the metal mesh substrate 38 through aliquid solution or dispersion of a sealing material prior to laminationwith a reactive layer 34. In this aspect of the present invention, thesealing material is a Teflon® dispersion in water.

The reactive layers 34 with seal coating either applied as a dot patternon inner facing major surfaces or heat seal netting material (not shown)interposed are juxtaposed to the current collecting substrate 32 in a"sandwich" of layers, and the sandwich subjected to bonding heat andpressure sufficient to activate the heat seal coating to bond thereactive layers 34 to the metal substrate 32. The materials of theelectrochemical cathode are selected such that activation temperaturesof the heat-seal coating to bond the layers does not damage any layersor component substance, generally a temperature less than 250° F. Thehydrophyllic layer 36 may be simultaneously laminated to one of thereactive layers 34 as part of the laminate sandwich, or in a separatebut like bonding step. Once the layers are bonded, the resultinglaminate is cut to size to provide the individual electrochemicalcathodes.

The process of the present invention is set forth in the followingspecific examples which are intended to be merely illustrative and thepresent invention is intended not to be limited thereto.

EXAMPLE I

A conductive nonwoven web of carbon fibers (International PaperCompany), is introduced into a continuous web machine to effect coatingand drying operations. The impregnated mixture (carbon/Teflon®) iscomprised of 36 gms. of an aqueous dispersion of Black Pearls (carbon)2000 (15% solids) and 0.9 gms. Teflon® T-30 (60% solids) applied at asolids content of 12.0%. Impregnation is effected to provide an add-onof 60 GSM. The resulting impregnated layers and fine nickel mesh(Delker) precoated with an adhesive are passed through an ovenmaintained at a temperature of from 200° F. to 300° F. and thence weband passed through laminating rolls at nip pressure of 600 pounds perlineal inch at a temperature of 250° F. to form an electrochemicalcathode suitable for use in an electrochemical battery.

EXAMPLE II

The procedure set forth in Example I is similarly followed except that3.2 gms of Teflon® T.30 and 18 gms of a 0.4% solution of potassiumpermanganate (12% based on solids content) is added to the impregnationmixture and is observed to react within a few minutes. The resultingcathode had a polarization voltage at least 10% less than a non-catalystsystem and is particularly suitable for use in a lithium electrochemicalbattery.

EXAMPLE III

The procedures set forth in both EXAMPLES I and II similarly followedexcept that a hydrophobic polymeric (Teflon®) film is included in thesandwich to be laminated, i.e. included against a side of one of theimpregnated webs to form a four layered electrochemical cathode suitablefor use in the laminate of each Example is preheated at 180° F. with themicroporous polymeric film and passed through laminating rolls at atemperature of 200° F.

EXAMPLE IV

The procedure as set forth in EXAMPLE III is similarly followed exceptan embossed roll is used and the laminate sandwich is resented so thatthe hydrophobic film comes into contact with the polymeric film.

While the invention has been described in connection with an exemplaryembodiment thereof, it will be understood that many modifications willbe apparent to those of ordinary skill in the art; and that thisapplication is intended to cover any adaptations of variations thereof.Therefore, it is manifestly intended that this invention be only limitedby the claims and the equivalents thereof.

What is claimed:
 1. An electrochemical cathode, which comprises:acurrent collecting substrate capable of being connected to electricalcircuitry; and conductive nonwoven webs laminated to said currentcollecting substrate and impregnated with a mixture of carbon particlesand a nonfibrous polymeric substance for holding said carbon particlesin said web, said conductive nonwoven web being formed of conductivecarbon fibers.
 2. The electrochemical cathode as defined in claim 1 andfurther including a hydrophobic polymeric film bonded to one of saidconductive nonwoven webs.
 3. The electrochemical cathode as defined inclaim 1 or 2 wherein said carbon fibers are of a diameter of from about5.0 to 15.0μ.
 4. The electrochemical cathode as defined in claim 3wherein said carbon fibers are of a length of from about 0.5 to 1.25inches.
 5. The electrochemical cathode as defined in claim 1 or 2wherein said carbon fibers of said conductive nonwoven web exhibit aresistance of less than about 20 ohms/sq.
 6. The electrochemical cathodeas defined in claim 1 or 2 wherein said conductive nonwoven web providesa front-to-back electrical conductivity of less than about 1 ohm.
 7. Theelectrochemical cathode as defined in claim 1 or 2 wherein said carbonparticles have a surface area greater than about 1000 m.² /g.
 8. Theelectrochemical cathode as defined in claim 1 or 2 wherein said currentcollecting substrate is metal mesh.
 9. The electrochemical cathode asdefined in claim 1 or 2 wherein said current collecting substrate isbonded by a sealing material to said conductive nonwoven webs.
 10. Theelectrochemical cathode as defined in claim 9 wherein said sealingmaterial is a dispersion of PTFE.
 11. The electrochemical cathode asdefined in claim 2 wherein said hydrophobic polymeric film is embossedonto said conductive nonwoven web.
 12. The electrochemical cathode asdefined in claim 2 or 11 wherein said hydrophobic polymeric film isformed of polytetrafluoroethylene.
 13. The electrochemical cathode asdefined in claim 1 or 2 wherein said mixture includes a catalyst. 14.The electrochemical cathode as defined in claim 13 wherein said catalystis selected from the group consisting of potassium permanganate andsilver oxide.
 15. A process for forming an electrochemical cathode,which comprises:(a) impregnating a conductive nonwoven fibrous web witha dispersion comprised of carbon particles and a nonfibrous polymericmaterial for holding said carbon particles to said web. (b) laminating acurrent collecting substrate between layers of said impregnated web toform said electrochemical cathode.
 16. The process for forming anelectrochemical cathode as defined in claim 15 and further including thesteps of forming and drying a sandwich of said current collectingsubstrate and said layers of said impregnated web prior to step (b). 17.The process for forming an electrochemical cathode as defined in claim16 wherein drying is effected at a temperature of from 200° F. to 300°F.
 18. The process for forming an electrochemical cathode as defined inclaim 15, 16 or 17 wherein said current collecting substrate is coatedwith a polytetrafluoroethylene dispersion.
 19. The process for formingan electrochemical cathode as defined in claim 18, wherein laminating iseffected at a temperature of about 150° F. to about 400° F.
 20. Theprocess for forming an electrochemical cathode as defined in claim 15wherein said nonfibrous polymeric material is polytetrafluoroethylene.21. The process for forming an electrochemical cathode as defined inclaim 15 and further including a polymeric film in said sandwich incontact with one of said layers of said impregnated web.
 22. The processfor forming an electrochemical cathode as defined in claim 16 andfurther including a polymeric film in said sandwich in contact with oneof said layers of said impregnated web.
 23. The process for forming anelectrochemical cathode as defined in claim 21 or 22 wherein step (b) iseffected with an embossing roller in contact with said polymeric film.24. The process for forming an electrochemical cathode as defined inclaim 21 or 22 wherein said current collecting substrate is coated witha polytetrafluoroethylene dispersion prior to step (b).
 25. The processfor forming an electrochemical cathode as defined in claim 24 whereindrying is effected at a temperature of from 200° F. to 300° F.
 26. Theprocess for forming an electrochemical cathode as defined in claim 25wherein step (b) is effected with an embossing roller in contact withsaid polymeric film.